As of Wednesday morning, The Hill's whip list had three far-right Republicans on-record as planning to vote against the repeal-and-replace legislation: Justin Amash of Michigan; David Brat of Virginia; and Jim Jordan of Ohio. The positions of another 12 GOP representatives were described as "unclear or uncertain."

Right-wing Freedom Caucus member Rep. Mo Brooks (R-Ala.), one of those listed as on-the-fence, went as far as to say Tuesday evening: "Right now the Speaker of the House does not have the votes to pass this bill unless he's got substantial Democratic support."

The bill "faces perhaps an even steeper climb in the Senate," The Hill reported, where "at least eight Republican senators have voiced concerns with aspects of the legislation."

"[T]he margin for error is slim," the outlet noted. "Assuming all Democrats vote against the legislation, GOP leaders cannot afford more than 21 defections in the House and two in the Senate."

Meanwhile, two House committees are taking up the bill on Wednesday morning—though they still do not have a "score" on the legislation from the Congressional Budget Office (CBO), making it impossible for lawmakers (or the public) to know its human or financial costs.

That's a big problem, and not just for Democrats like House Minority Leader Nancy Pelosi (Calif.), who on Tuesday demanded of Ryan that no votes be held—in committees or on the House floor—until that score is released. Politicoreports that Senate Republicans have similarly said that until they have a CBO score, "it's unlikely many lawmakers will sign on. Privately, some Republicans fear the CBO score will show that millions of Americans will lose coverage. The CBO is expected to weigh in later this week."

Still, without many hardcore conservatives on the House Ways and Means or Energy and Commerce committees, it's likely the mark-ups will proceed apace. (Though Democrats do plan to "make some Republicans take some really tough votes" that are "revealing" about GOP's priorities on healthcare, according toCNN, which cited a Democratic aide.)

"The House floor is the problem," journalist David Dayen wrote on Twitter. He later noted that Breitbart's opposition to the legislation was a signal that "it's over" for "Ryancare."

At Vox, reporter Andrew Prokop outlined the three groups that could potentially "scuttle" the AHCA when it goes before the full chamber:

The far right of the Republican Party. Within the GOP, staunch conservative politicians and groups have objected most vocally to the bill so far, because they're upset that it doesn't do enough to roll back Obamacare. Heritage Action, the Club for Growth, Freedomworks, Americans for Prosperity, and Freedom Partners have already declared their opposition to the current draft. Sens. Rand Paul (R-Ky.) and Mike Lee (R-Utah), and several members of the House Freedom Caucus, have joined them.

Mainstream members who listen to lobbyists.Many of the more moderate Republicans in the House and Senate are receptive to lobbying from important interest groups. So if associations representing doctors, hospitals, or insurers say the bill has problems, the House could well slam the brakes. (And lo and behold, on Tuesday afternoon, the American Hospital Association wrote that it "cannot support the American Health Care Act in its current form.")

Vulnerable members. Swing district representatives are already blanching at the prospect of a tough reelection fight. These are the members who really have their careers on the line, and several of them expressed misgivings about repeal and replace in leaked audio from a congressional retreat in January.